This invention relates to methods and apparatus for reducing noise breakthrough, errors or distortion in the output signal obtained from a demodulator fed with a frequency or phase modulated input signal having a comparatively low carrier-to-noise ratio. The invention is particularly applicable to the demodulation of FM television signals transmitted by satellites having for example a transmission frequency of 10 to 12 GHz and a bandwidth of 36 MHz. For economy and convenience such signals are received using small aperture dish antennas with the result that received carrier-to-noise ratios are typically as low as 2 dB to 10 dB. However, the invention is also of particular use in FM speech reception and low rate data communications as used, for example, in paging systems.
Conventional FM demodulators are often based on a delay line discriminator having a limiter in the intermediate frequency (IF) stages, a delay line coupled to the output of the limiter with delay T equivalent to a phase shift of 90.degree. at the intermediate frequency, a multiplying device in the form of a mixer coupled to multiply the signal fed to the delay line by the signal obtained from the output of the delay line, and a low pass filter coupled to the output of the mixer. The response (output voltage versus carrier frequency f.sub.c) of this discriminator when supplied with an input signal of .vertline.A(t).vertline.(2.pi.f.sub.c t+.phi.) has a straight line characteristic between f.sub.c =f.sub.o -1/4T and f.sub.c =f.sub.o +1/4T, where f.sub.o is the IF center frequency. The value of the output depends only on the value of f.sub.c and is independent of the phase constant .phi. and the magnitude variation .vertline.A(t).vertline., the latter being removed by the action of the limiter. At the center frequency f.sub.o, the phase shift due to the delay T is (i.2.pi.+.pi./4) where i is an integer having any positive value (0, 1, 2, 3, etc.). In TV satellite receiving systems the value of T may be in the range 5 to 10 ns.
At low input carrier-to-noise ratios, i.e. 10 db or less, noise breakthrough is caused by input noise instantaneously being able to determine the polarity of the signal input to the limiter, causing an impulse noise spike at the output. Due to this effect, impulse noise spikes are noticeable when the input signal carrier-to-noise ratio is at or below a certain threshold.
It is an object of this invention to provide a demodulator arrangement which is less sensitive to input noise so that, in the case of a satellite receiving system, a smaller dish antenna can be used for a given received signal carrier-to-noise ratio.